1. Field Of The Invention
This invention relates to a method of and apparatus for recovering high grade gold alloy in an unoxidized condition from karat gold clad to a base metal substrate, such as, for example, nickel, silver, brass, copper and the like. More specifically, the invention relates to a method and apparatus of such type wherein the recovery of gold is effected without significant oxidation of the base metal substrate, under electrolysis conditions.
2. Description Of The Prior Art
In a general practice of the metals plating, jewelry and electronic circuitry arts, it has been common practice to reclaim precious metal values from scrap circuitry, used coated articles, scrap jewelry and the like. This has been particularly true in the case of gold, despite fluctuations in recent years in the market value thereof, and the prior art has proposed a great number of approaches for recovering this precious metal for reuse. Despite the number of possible processes heretofore employed or proposed in the art, all such methods have been characterized by various deficiencies which have limited their applicability or desirability in use.
U.S. Pat. No. 1,996,985 discloses a method for processing goldsmiths' waste residues containing precious metals such as gold, silver and platinum in addition to base metals such as copper, nickel and zinc. The residue precious metal-containing substrate according to the disclosed process is subjected directly to electrolysis in an electrolyte which is either faintly acid, neutral or slightly alkaline and contains salts of nitric acid. An exemplary solution for such purpose is copper nitrate faintly acidified by nitric acid in aqueous solution. As a result of such electrolysis, sludges are deposited on the cathode and anode, the cathodic sludge comprising metallic silver and copper, together with basic copper compounds, from which silver may be recovered in pure form by leaching of the cathodic sludge with diluted sulfuric acid; the anodic sludge is removed by scraping or otherwise from the anodes and contains the gold and platinum values of the substrate under treatment along with certain quantities of silver and some base metals. The anodic sludges are treated by extraction with nitric acid or sulfuric acid at elevated temperature to obtain a residue which is nearly free from silver and copper. The patent discloses that boiling of the anodic sludge in nitric acid is a suitable method for recovery of the crude gold.
The above-described prior art method of gold recovery suffers from several disadvantages. First, it is necessary to prepare an anode from the substrate metals by fusing and casting into a slab the discrete scrapings, sweepings, sludges or residues of the gold-containing material. Second, the electrolysis employed to recover the gold values includes a relatively high current density, which is disclosed as ranging from about 350 amperes per square meter at the anode, up to values on the order of 1,000 to 1,500 amperes per square meter in the case of materials which are relatively poor in precious metals content. Third, the recovery of gold from the anodic sludge involves boiling of the sludge in nitric or sulfuric acid, thus entailing handling and disposal problems associated with such high acid concentrations.
U.S. Pat. No. 2,185,858 discloses a method of removing gold, silver and/or palladium from a metal base material such as zinc, copper, brass or bronze. In the case of gold-clad metals, the substrate is made an anode in an electrolyte comprising a solution of concentrated sulfuric acid, wherein a cathode is disposed which is formed of nickel or other suitable cathode material. Upon electrolysis being initiated the sulfuric acid adjacent the anode is disclosed as being converted into persulfuric acid which dissolves the gold from the anode, but which passes from the immediate vicinity of the anode and then precipitates out from the acid and collects on the bottom of the electrolysis tank as substantially pure gold sludge. In order to minimize attack of the acid on the substrate metal, particularly in the case of brass, it is disclosed to maintain temperature in the electrolysis bath below 38.degree. C. The disadvantages associated with this method of recovering gold again include the use of highly concentrated acids, as well as the fact that the gold, being deposited in the electrolysis solution in the form of a precipitated sludge, must be recovered from the electrolysis solution, and such recovery is subject to inefficiencies from material losses of the desired product gold.
U.S. Pat. No. 2,735,810 discloses a method for recovering gold from gold-clad material, which may contain other metals such as nickel, iron, tungsten, molybdenum and cadmium, wherein the clad substrate is subjected to electrolysis in a bath containing from about 3 to about 16% alkaline metal or ammonium ferrocyanide and from about 0.25% to about 8% alkali metal or ammonium cyanide. The electrolysis bath is maintained at a temperature of from about 20.degree. C. to 80.degree. C. (68.degree. F. to 176.degree. F.) and a pH of from about 7.5 to about 12. The ratio of ferrocyanide to cyanide in the bath is disclosed as being within the range of from 2:1 to 60:1. In this system the scrap metal functions as the anode and gold is deposited on the cathode, from which it may be scraped or otherwise removed. A current density of from 1 to 6 amperes per square foot is thought to have been satisfactorily employed. Among the problems associated with the use of this method, is the fact that disposal of the spent electrolysis bath requires treatment such as by chlorination in order to provide an effluent quality dischargeable stream from the process system. Further, the use of ferrocyanide and cyanide constituents in the electrolysis bath raises the danger of toxicity arising from hydrogen cyanamide generation during the carrying out of electrolysis. Further, the gold which is withdrawn from the anodic substrate deposits on the cathode, from which it must be scraped or otherwise mechanically removed, unless the cathode is gold or other substrate metal suitable for use in conjunction with the recovered gold.
In U.S. Pat. No. 3,663,388, there is disclosed a process for removing gold coating from a substrate, wherein the substrate is subjected to electrolysis to oxidize the gold coating under low voltage conditions, such as less than about 20 volts DC. The electrolysis solution may be a 1 molar (M) aqueous solution of an ammonium salt such as ammonium chloride and a 0.1 M solution of hydrochloric acid. The patent discloses that other salts may be substituted for ammonium chloride and other acids such as nitric acid may be used in place of the hydrochloric acid. In either case, the process is conducted to oxidize the gold. As the gold oxidizes, it blackens in color as the electrolysis solution functions as an etchant to etch the surface such that the oxidized gold adheres poorly to the substrate. Subsequently, the substrate may be removed from the electrolysis bath and the oxidized gold may be removed by wiping with a soft cloth or, alternatively, the etched substrate may be placed into an ultrasonic tank which vibrates the oxidized gold particles off the metal sheet. This gold removal process, as is also the case with various of the prior art methods described hereinabove, requires a two-step treatment of the gold-clad substrate. In the first step, the gold coating is oxidized and in the second step the oxidized coating is mechanically removed from the substrate, whereas in other prior art systems the electrodeposited gold must be mechanically or otherwise removed from one of the electrodes in the electrolysis system.
In addition, the gold product in this system is in the form of an oxide which is unsuitable in many applications for reuse without further treatment to reduce the gold oxide to pure metallic gold product.
U.S. Pat. No. 4,426,225 discloses a method for recovering gold in flake form from substrate materials wherein the gold is deposited on an intermediate layer of copper, nickel, tin and alloys thereof, in which the base of the substrate is insoluble in the method of the invention. The disclosed method comprises the first step of contacting the gold-containing scrap material with an aqueous nitric acid etching solution, which functions by preferentially etching away the intermediate metallic layer, effectively chemically tunneling under the gold coating. As a result, the gold coating has progressively loosened and flaked off into the etching solution. This first step produces a pregnant etching solution comprising gold flakes in a liquor comprising nitrates of the metals comprising the underplating. The second step of physically separating the gold flake from the pregnant etched liquor is carried out by any suitable conventional means such as filtration or centrifugal separation. The etchant solution employed in the method of this patent is approximately 50% by weight nitric acid in aqueous solution. It is also disclosed to employ additives such as phosphates and citric acid to enhance the recovery of gold by providing a frothing action that overcomes surface tension induced by the adherence of gold flake to the substrate. The etching step of this process is suitably carried out for a period of time on the order of 2 hours to 4 hours at ambient temperature. Among the disadvantages of this method are the facts that high concentrations of acid are required and the treatment time for the etching step is comparatively long in duration.
In the gold recovery process disclosed in U.S. Pat. No. 4,462,879, gold or gold scrap is melted and inquarted using copper or other non-ferrous metals or, in the case of platiniferous gold, silver. The resultant gold copper or gold silver alloy slab is employed as an anode in an electrolytic cell employing an aqueous nitric acid solution, wherein the electrolysis solution contains 28% by weight nitric acid, in addition to a disodium salt of ethylenedinitrilotetraacetic acid (EADS) which functions as a detergent or emulsifying agent preventing refined gold particles falling from the anode during electrolysis from floating in the electrolyte solution, and also prevents polarization of the anodes which is stated to impede the process. The electrolytic conditions in this system includes a current density of 2-2.5 amps per square inch of anode surface, at a voltage of 3-5 volts. As a result of the electrolysis being carried out, the base metal such as copper is dissolved in the electrolyte solution, concurrent with the gold in the anodic body precipitating to the bottom of the container as a mud or slime residue. The electrolytic solution then is filtered from the slime which is boiled in concentrated nitric acid or washed with ammonia hydroxide. The resultant gold mud is again filtered, washed thoroughly with distilled water, dried and heated to a molten state at a purity on the order of 99.9%. Among the disadvantages of this process are again a relatively high concentration of nitric acid in the electrolytic solution, and a requirement of boiling the gold slime in concentrated nitric acid or ammonia hydroxide, either of which must be neutralized for disposal.
In summary, the methods of the prior art in general require highly concentrated acid solutions for etching or electrolysis removal of gold from a substrate or else mechanical removal such as by scraping from an electrode on which the gold is deposited or else reconcentration and purification where gold is deposited as a precipitate in the etching or electrolysis solution. As indicated, various of the prior art methods require considerable time for recovery of the desired product gold and in others the substrate is substantially oxidized or dissolved into solution, with the attendant requirement of treatment to precipitate or otherwise concentrate such base metal component of the spent electrolytic bath liquor.
In this connection, it is to be noted that in many applications it is desired to simply remove the gold cladding from the substrate without adverse effect on the character of the substrate, such as oxidation or dissolution thereof, so that the substrate may be directly utilized after the treatment step for gold recovery. This is particularly the case in jewelry or microelectronic circuitry systems, wherein the gold may be plated on a substrate metal such as nickel or silver-coated nickel and the base metal thus has significant reuse value so that any deterioration or oxidation thereof is detrimental to such reclamation of it.